Microwave first 3 ingredients in a 2-quart microwave-safe bowl at HIGH 1 1/2 minutes or until butter and chocolate melt, stirring twice.
Stir together 1 cup melted butter and brown sugar. Add eggs, stirring until blended. Stir in chocolate mixture, flour, and next 3 ingredients. Pour into a greased 15- x 10-inch jellyroll pan.
Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean (do not overbake). Cool in pan on a wire rack; cut into bars. Serve with ice cream, if desired.
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